TCP is a commonly used communication protocol for communicating packet data from one data processing system to another over a data network. Establishing a TCP session (also called a TCP connection) between two data processing systems involves a multi-step exchange of preliminary information between the two systems. This multi-step exchange of information is called a three-way handshake.
A TCP session is terminated, or the TCP connection is broken when one of the two participating data processing systems sends a reset packet (TCP RST packet, RST packet) to the other participating data processing system. For example, if during the session, one data processing system fails to receive one or more acknowledgements from the other data processing system in the session, or the connection becomes unsynchronized for this or other reason, the data processing system sends a reset packet to the other data processing system. In some cases, a data processing system may receive a packet from another data processing system when the data processing system is not participating in a session with the other data processing system. The data processing system sends a reset packet to the other data processing system under such circumstances as well. There are many conditions, circumstances, or reasons, including but not limited to those described above, why one data processing system would sends a reset packet to another data processing system during TCP/IP communications.